1. Field of the Invention
The present invention relates an alkaline manganese cell. More particularly, the present invention relates to an improvement of a binder to be contained in a positive electrode composition of an alkaline manganese cell.
2. Description of the Related Art
Hitherto, to improve the ease of weighing and moldability, a positive electrode composition of an alkaline manganese cell is prepared by wet mixing positive electrode components including manganese dioxide as a positive electrode active material and graphite powder or carbon black as an electrically conductive filler, extruding a resulting mixture with an extruder having a die with extrusion nozzles of 0.5 to 1.0 mm in diameter to produce granules, and sieving the granules to adjust a granule size distribution. In the above process, a water-soluble binder having gluing properties such as polysodium acrylate or carboxymethylcellulose is added to the mixture of positive electrode components to bind the components (cf, for example, Japanese Patent Kokai Publication No. 2266/1986).
However, since the water-soluble binder absorbs an electrolytic liquid, the molded positive electrode composition absorbs the electrolytic liquid, swells and softens. Therefore, adhesion of the positive electrode composition to a positive electrode can decreases, electron conductivity through the electrically conductive filler in the positive electrode composition decreases so that internal resistance increases, or the electrolytic liquid is absorbed by the positive electrode composition and migrates towards the positive electrode side so that an amount of the electrolytic liquid near a zinc negative electrode active material decreases, whereby a discharge reaction does not proceed sufficiently and a discharge time of the cell is shortened. Due to the above phenomena, discharge performance of the alkali manganese cell is deteriorated. Since the wet strength of swelled molded positive electrode composition decreases as time passes, the cell performance remarkably decreases, in particular, during storage.
It has been proposed to use low molecular weight polytetrafluoroethylene powder as a binder for the positive electrode composition in an amount of 0.3 to 3% by weight based on the total weight of manganese dioxide and the electrically conductive filler to suppress an increase of the internal resistance due to the absorbance of the electrolytic liquid with the positive electrode composition and decrease of discharge performance due to migration of the electrolytic liquid towards the positive electrode composition (cf. Japanese Patent Kokai Publication No. 220373/1989).
That is, the manganese dioxide powder, which is the positive electrode active material, the electrically conductive filler, such as graphite or carbon black, and the low molecular weight polytetrafluoroethylene powder are mixed with a small amount of an aqueous solution of potassium hydroxide or sodium hydroxide which is used to neutralize acidity due to manganese dioxide and prevent the corrosion of metals with strong oxidation by manganese dioxide, and water. Then, the resulting mixture is extruded with an extruder to form granulates and dried to adjust a water content in a range in which good moldability and easy weighing are achieved. The granules are charged in a mold and molded in a suitable form such as a ring. The formed positive electrode composition is placed in a positive electrode can to assemble the alkali manganese cell. The low molecular weight polytetrafluoroethylene powder absorbs a smaller amount of the electrolytic liquid than the conventional binder, such as polysodium acrylate or carboxymethylcellulose. Accordingly, deterioration of adhesion of the positive electrode composition to the positive electrode can be caused by excessive absorption of the electrolytic liquid by the positive electrode composition, and the decrease of electron conductivity in the positive electrode composition can be prevented. Since the electrolytic liquid on the negative electrode side is not excessively absorbed by the positive electrode composition, a sufficient amount of the electrolytic liquid is kept near the zinc negative electrode active material and the discharge reaction proceed sufficiently. Therefore, the discharge performance is not deteriorated.
By the use of low molecular weight polytetrafluoroethylene powder as the binder of the positive electrode composition, the components of the positive electrode composition are well bound, and powdering of the granules and disintegration of the molded positive electrode composition in the cell are prevented. However, since the low molecular weight polytetrafluoroethylene powder does not contribute to the discharge reaction, its addition reduces the amount of the positive electrode active material to be contained in the positive electrode composition, whereby the discharge capacity of the cell decreases. In addition, since the polytetrafluoroethylene powder has no conductivity, the electron conductivity in the positive electrode composition is decreased.